Process for coloring the surface of aluminum workpieces by anodization

ABSTRACT

This invention relates to the coloring of the surface of aluminum workpieces by anodization process wherein the anolyte consists principally of maleic acid with a small proportion of sulfuric acid for conductivity, and the catholyte consists of an organic acid of good conductivity or a mixture of organic acid to which a small quantity of sulfuric acid is added to conductivity. The anolyte and the catholyte are separated by a porous, ionpermeable diaphragm.

United States Patent Sautter et al.

Nov. 6, 1973 l l l l PROCESS FOR COLORING THE SURFACE OF ALUMINUMWORKPIECES BY ANODIZATION Inventors: Werner Sautter, Bonn; Ahmad Gilak,Bonn-Beuel, both of Germany Vereinigte Aluminium-WerkeAktiengesellschaft, Bonn, Germany Filed: Oct. 8, 1970 Appl. No.: 79,242

Assignee:

Foreign Application Priority Data Oct. 8, [969 Germany P 19 50 716.5

US. Cl. 204/58 Int. Cl C23b 9/02 Field of Search 204/58 References CitedUNITED STATES PATENTS 1/1972 Coates 204/58 Primary Examiner-John H. MackAssistant ExaminerR. L. Andrews Attorney-Michael S. Striker [57]ABSTRACT This invention relates to the coloring of the surface ofaluminum workpieces by anodization process wherein the anolyte consistsprincipally of maleic acid with a small proportion of sulfuric acid forconductivity, and the catholyte consists of an organic acid of goodconductivity or a mixture of organic acid to which a small quantity ofsulfuric acid is added to conductivity. The anolyte and the catholyteare separated by a porous, ion-permeable diaphragm.

4 Claims, 1 Drawing Figure PROCESS FOR COLORING THE SURFACE OF ALUMINUMWORKPIECES BY ANODIZATION BACKGROUND OF THE INVENTION Colored oxidelayers of aluminum can be prepared in a variety of known electrolytesconsisting of aqueous solutions of sulfonic acids such as sulfosalicylicacid and sulfophthalic acid, or of dicarboxylic acids with smalladditions of sulfuric acid. Maleic acid is a member of the last groupand is frequently used as the electrolyte with additions of oxalic andsulfuric acids because of its low cost and the uniformity of theobtainable colors at relatively low voltages. The colors range fromlight brown to dark gray and to black.

A specific disadvantage of using the unsaturated maleic acid as a majorcomponent of the electrolyte results from the fact that the hydrogengiven off at the cathode reduces maleic acid to succinic acid. Thismaterial has a low solubility and consequently it crystallizes out atthe opening of the necessary compressed air circulator and on theequally necessary cooling coils. As a result of this difficulty, thebath after throughput of to din of aluminum surface per liter ofsolution must be regenerated, this being carried out by cooling theentire bath or at least a substantial portion thereof to 10 to C incontrast with the usual operating temperature' which is 20 to 30C andremoval of the crystallized out succinic acid. Moreover, the originalconcentration of the electrolyte must be maintained by addition ofsuitable quantities of maleic acid.

SUMMARY OF THE INVENTION It is an object of the present invention todevelop a process for coloring the surface of aluminum workpieces whileavoiding the difficulties of loss of solute by reduction andcrystallization at the cathode.

It is a further object of the present invention to develop a method ofanodizing aluminum surface to produce colors ranging from light brownthrough dark gray to black.

According to the present invention, the formation of succinic acid frommaleic acid can be avoided by separating the cathode space and the anodespace. This is carried out, preferably, by enclosing the cathode in aboxed-shaped or tube-shaped enclosure, the wall of which consists of afine-pored, ion-permeable material and which contains an electrolytewith a solute consisting of an organic acid with good electricalconductivity, or a mixture of an organic acid with a small quantity ofsulfuric acid.

A suitable container for the cathode is a mixture based on fine aluminumsilicates fired at high temperature where the pore diameters are in theneighborhood of 1 micron. At a wall thickness between preferably A; and2/5 inch, such a container has adequate mechanical strength and at thesame time a low electrical resistance. The surface area of the containeror the surface areas of a battery of parallel-connected containers is soregulated that a current density of about 3 A/dm is not exceeded.

The concentration of the electrolyte within the cathode container shouldpreferably be so high that at worst only a small quantity of maleic acidcan diffuse thereinto. Suitable mixtures of electrolyte are for example,3 to 8 wt. percent of oxalic acid and 0.2 to 1 weight percent sulfuricacid. Another suitable combination is 5 to 30 wt. percent citric acid orsulfosalicylic acid with an addition of sulfuric acid in the samequantity. Such electrolyte compositions have additional numerousadvantages such as the fact that in the event of damage to thecontainer, no essential change in the properties of the electrolyte inthe anode chamber occurs. The composition of the anolyte may be, forexample, 20 weight percent of maleic acid, 1 weight percent of oxalicacid and 0.4 weight percent of sulfuric acid with the remainder beingwater.

A further advantage of the process according to the present inventionlies in the fact that the aluminum which goes into solution as a resultof the anodic oxidation, which according to the present state of the artis removed by ion exchange, in this case collects in the cathodecontainer without any hindrance to the anodic oxidation process. Thelifetime of such a bath is thereby enhanced.

By operating according to the present invention, it becomes possible tocolor anodize parts made of pure aluminum as well as parts made ofaluminum alloys specially adapted for color anodization, in electrolytesbased on maleic acid, at substantially lower costs than was possiblewith previous techniques of anodization. The throughput withoutregeneration of the bath is from 10 to 50 times greater than waspreviously possible. The requirement for periodic additions of maleicacid now depends only on replacement of drag-out losses.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE shows schematicallythe arrangement of an anodization cell according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An arrangement for carrying outthe process in accordance with the present invention is shown in thedrawing schematically. The anode l which is the object to be suppliedwith a colored coating is placed in a vessel and is separated from acathode 2 by means of a diaphragm 4. The numeral 3 indicates the aqueouselectrolyte which, for example, may consist of 20 weight percent ofmaleic acid, 1 weight percent of oxalic acid, and 0.4 weight percent ofsulfuric acid with the remainder being water. The electrolyte within thediaphragm is indicated by the numeral 5. The cathode 2 is shown to beperforated as indicated at 6; the objective is to permit hydrogen toenter into the inside of the cathode and to escape at the top.

The following examples illustrate the advantages accruing from theoperation according to the present invention. The first exampleillustrates the results obtained when operating according to the priorart.

EXAMPLE I Anodization of an extruded shape consisting of AIM- gSi 0.5composition was carried outin a solution consisting of 20 weight percentmaleic acid, I weight percent oxalic acid, and 0.4 weight percent ofsulfuric acid. The anodization was continued until a dark brown colorresulted. The cathode sheet was made of aluminum which was placed indirect contact with the electrolyte. It was found to be necessary toregenerate the solution after a throughput of about 6 dmll of bathcontent.

EXAMPLE 2 A cathode of the same type as used in Example 1 was placed ina separate container containing an electrolyte consisting of 20 weightpercent citric acid, 0.4 weight percent of sulfuric acid; the anode wasplaced in a composition similar to that of Example 1. After a throughputof 100 dm ll of bath contents, only a small quantity of succinic acidhas been formed; this amount was completely harmless with respect tocontinuation of the anodization process. In the cathode space, after thesame period of time, the solution contained 8 percent of dissolvedsuccinic acid, and percent of succinic acid had crystallized out. Thisformation of succinic acid was a result of the inward diffusion ofmaleic acid. This quantity of transformed maleic acid had no efi'ectwhatsoever on the further color-anodization process.

A further effect was the increase of the aluminum content in the cathodespace; this amounts to about 40 percent of the aluminum which had goneinto the solution during the color anodization process. In contrast withthe processes previously used, the quantity of aluminum to be removed byion exchange was greatly decreased. Moreover, renewal of the smallquantity of solution contained in the cathode space made it possible toproceed with the color anodization as before. Also, by keeping thecurrent density at about 2 A/dm of surface area of the container, thevoltage required for operation was increased only by about 4 volts as aresult of the presence of the cathode container.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intendeded to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. A process for coloring the surface of an aluminum workpiece byanodization which comprises passing an electric current between thealuminum workpiece immersed as anode in an aqueous anodizing anolyteconsisting essentially of maleic acid containing a minor proportion ofsulfuric acid or oxalic acid or both, and a conductive cathode immersedin an aqueous catholyte which comprises an electrically conductiveorganic acid other than maleic acid or a mixture of the said organicacid together with a minor proportion of sulfuric acid, the said anolyteand catholyte being separated from each other by a porous ion-permeablediaphragm.

2. A process as defined in claim 1 in which the anolyte comprisesapproximately 20 percent by weight of maleic acid, approximately 1percent by weight of oxalic acid, and approximately 0.4 percent byweight of oxalic acid.

3. A process as defined in claim 1 in which the catholyte comprisesbetween 3 and 8 percent by weight of oxalic acid and between 0.2 and 1percent by weight of sulfuric acid.

4. A process as defined in claim 1 in which the organic acid in thecatholyte is oxalic, citric, sulfosalicylic, or sulfophthalic acid, inan amount equivalent to between 5 and 30 percent by weight.

2. A process as defined in claim 1 in which the anolyte comprisesapproximately 20 percent by weight of maleic acid, approximately 1percent by weight of oxalic acid, and approximately 0.4 percent byweight of oxalic acid.
 3. A process as defined in claim 1 in which thecatholyte comprises between 3 and 8 percent by weight of oxalic acid andbetween 0.2 and 1 percent by weight of sulfuric acid.
 4. A process asdefined in claim 1 in which the organic acid in the catholyte is oxalic,citric, sulfosalicylic, or sulfophthalic acid, in an amount equivalentto between 5 and 30 percent by weight.